- Email: [email protected]
Faecal pellets and energy flow in rivers
224
News & Comment
TRENDS in Ecology & Evolution Vol.16 No.5 May 2001
Journal Club
Vicious fig wasps in viscous populations Understanding when interacting individuals should conflict or cooperate is an enduring topic in evolutionary ecology. Cooperation can be favoured through kin selection when those interacting are genetic relatives. Because high levels of relatedness can be generated in ‘viscous’ populations, characterized by limited dispersal, it has often been assumed that population viscosity will lead to cooperation. However, the same viscosity might also increase resource competition between relatives, and recent theory predicts that this might oppose the evolution of cooperation. The predicted and opposing influences of relatedness and competition can be difficult to disentangle in practice but a new comparative study on male–male conflicts in fig wasps shows that fighting is correlated with resource competition rather than with genetic relatedness1. Fig wasps develop within sycones, which are the closed inflorescences often called ‘figs’. The females are always winged but males of many species are wingless and nondispersing. Wingless males mate with females inside the sycone, before the females disperse. Male–male aggression varies greatly among species: in some, males have a ‘combat morphology’, consisting of armour and large
mandibles, whereas, in others, males have no structures for fighting or for defence. The average relatedness of competing males also varies greatly among species, mainly as a result of varying numbers of ‘foundress’ mothers laying eggs into each sycone (when more mothers contribute offspring, average offspring relatedness decreases). West et al. used sex ratio (a known correlate of foundress number) as an index of relatedness and quantified conflict levels across 25 species by assessing physical damage sustained by males during the mating period (scoring 0.5 points for loss of an antenna and the maximum of eight points for decapitation). Controlling for phylogeny, they found no relationship between relatedness and conflict level, but there was a negative correlation between injury level and the mean number of females (mating resource) developing in a fig.
This study provides the first good support for the prediction that lack of dispersal before resource competition counteracts possible benefits of cooperating with kin. Instead, conflict levels appear to be determined by the importance of fighting for (and mating with) any particular female: when females are rare (small brood sizes) fighting is at its most intense. West et al.’s paper is valuable not only because it tests the alternative predictions but also because it emphasizes the need for empiricists to assess competition, as well as relatedness. Moreover, the scale at which competition operates is important. For fig wasps, the key biological scale is at the sycone level: without male dispersal from the sycone, it appears that kin selection is cancelled by resource competition. 1 West, S.A. et al. (2001) Testing Hamilton’s rule with competition between relatives. Nature 409, 510–513
Ian C.W. Hardy [email protected] James M. Cook [email protected]
Faecal pellets and energy flow in rivers With the ever-increasing need for water abstraction and power generation, the attention of many researchers interested in freshwater systems has turned to river flow management. However, as a new paper by Björn Malmqvist, Roger Wotton and Yixin Zhang shows, interest in management of anthropogenic changes to ecosystems does not preclude advancement in basic science. Blackfly larvae (Diptera: Simuliidae) are ubiquitous in most freshwater ecosystems, and their feeding habits result in an unusual transformation of organic matter. Along with several other filter-feeding macroinvertebrates, blackfly larvae remove fine particulate organic matter, and even dissolved organic matter, from the water column and, as a consequence of somewhat poor digestion efficiency, produce large numbers of faecal pellets (FPs). In some
ways, this process is analogous to the amalgamation of organic matter into ‘marine snow’ by marine suspension feeders. However, in flowing water, the process of amalgamation results in increased retention of carbon, whereas in the marine environment and lakes, it results in increased loss of carbon from the upper layers. In streams and rivers, the suspended organic matter captured but not assimilated by the blackfly larvae would otherwise be carried downstream. The ‘recycled’ organic matter is heavier than its constituent parts and so sinks more rapidly, which, in turn, means that the organic matter contained in the FPs is available to the large contingent of detritus feeders found in flowing waters. Malmqvist et al. show that if these FPs were not produced, the potential energy loss of organic material from large river systems
could be immense. For instance, they estimated that for a single river in the period of April 1999 to August 1999, the average daily transport of organic matter in the form of FPs was 69.2 tonnes (dry mass), equivalent to 2.7 tonnes of carbon per day. The authors also sampled a river that had been dammed, and suggest that regulation of rivers by damming could considerably reduce the concentration of FPs in the water, either through negative impacts on blackfly populations via reduction of available habitat, or perhaps through reductions in flow, resulting in more rapid sedimentation of the FPs. However, in terms of carbon retention, these two processes have conflicting effects: loss of habitat means carbon is lost from the system, whereas reduced flow could result in increased carbon retention through increased sedimentation. Hence, the effect of damming on carbon flow is
http://tree.trends.com 0169–5347/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.
News & Comment
difficult to predict and likely to vary between and within rivers. If increased sedimentation of FPs turns out to be the main factor at work in regulated rivers, then the implications for the energy flow and functioning of these systems is unclear. Locally, increased settling of FPs into reservoirs might act as a sink – locking up the organic matter, and possibly contributing to increased methane release. Alternatively,
TRENDS in Ecology & Evolution Vol.16 No.5 May 2001
the flux of particulate organic matter available to detritivores in rivers might increase at the expense of those in estuarine systems. Either way, regulation of rivers would seem to be redirecting a large portion of the energy flow through river systems and should receive more attention, both from those interested in management of perturbed systems and those involved with basic ecosystem functions.
225
1 Malmqvist, B. et al. (2001) Suspension feeders transform massive amounts of seston in large northern rivers. Oikos 92, 35–43
Stuart Humphries [email protected] Graeme D. Ruxton [email protected]
Inbreeding in nature: brothers and sisters, do not unite! Inbreeding, or mating among related individuals, causes problems with viability and fertility in small captive populations. Young inbred ungulates often die in zoos, and the eggs of inbred fruit flies frequently fail to hatch in the laboratory. But what is the role of inbreeding in nature? Does it really affect patterns of population change in the wild, and could it even cause extinctions? These questions were the subject of hot debate in the 1970s. By the late 1980s, many ecologists seemed to agree that other ecological factors were more important than inbreeding in reducing population size and in extinguishing populations. However, we could now be witnessing the revival of interest in inbreeding as a factor to contend with. Whereas theoretical studies have demonstrated how the accumulation of mildly deleterious alleles can lower the fitness of small populations, experiments, both in the laboratory and in the wild, have demonstrated significant effects of inbreeding on the density of several animal and plant species. Observations in 1998 suggested that inbreeding increases the risk of population extinction in the butterfly Melitaea cinxia1. Some of the same authors now report experimental evidence2 that not only confirms the earlier results, but also demonstrates when and how inbreeding
exerts its harmful effect. Working on small populations of Melitaea cinxia in the laboratory, Nieminen et al. found that inbreeding lowered the proportion of eggs hatched and decreased the survival of overwintering larvae. The larvae of this butterfly spin communal winter nests, but inbreeding made them spin nests full of holes, which failed to protect them from the elements. And when Nieminen et al. introduced in- and outbred populations into the field, they found that inbred populations had a much higher risk of extinction than did outbred ones. This study suggests that inbreeding in Melitaea cinxia may reduce local survival to the point of local extinction. And as the species occurs as a set of classical
metapopulations, where the extinction of small local populations is counterbalanced by the colonization of empty sites, an increased risk of local extinction might ultimately threaten the survival of the species at a landscape level. The work of Nieminen et al. is clearly a case study on a single species with many special features. In their study region, Melitaea cinxia typically occurs as very small local populations, where inbreeding between close relatives is more the rule than an exception. This should keep us from generalizing the current results too broadly. But by identifying inbreeding as a key agent in the dynamics of at least this species, Nieminen et al. do offer an eye opener. With more and more species being split into smaller and more fragmented populations, perhaps inbreeding might soon be a threat to the survival of many wild populations? 1 Saccheri, I. et al. (1998) Inbreeding and extinction in a butterfly metapopulation. Nature 392, 491–494 2 Nieminen, M. et al. (2001) Experimental confirmation that inbreeding depression increases extinction risk in butterfly populations. Am. Nat. 157, 237–244
Tomas Roslin [email protected]
Landscape dynamics can accelerate metapopulation extinction In metapopulations, local populations can be founded in, and go extinct from, distinct patches of suitable habitat. Previously, metapopulation models assumed static landscapes, where suitable habitat patches neither were created nor went extinct. Dynamics and persistence of such metapopulations were governed by the interaction between the life history of species, which determines the rates of
local extinction and colonization, and landscape properties. Metapopulation persistence was more probable for a larger amount of suitable habitat, and especially so, if suitable patches were connected. However, for many species, landscapes are highly dynamic rather than static. Therefore, Juan Keymer and co-workers conducted a new study of metapopulation persistence with a dynamic landscape
model where suitability of habitat patches changes over time1. Keymer et al. confirm that metapopulation persistence depends on the life-history of a species and on the amount of available habitat. Instead of purely spatial patch connectivity, however, spatio-temporal patch connectivity now becomes important. So far, at low connectivity, local dispersal was not
http://tree.trends.com 0169–5347/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.
News & Comment
TRENDS in Ecology & Evolution Vol.16 No.5 May 2001
Journal Club
Vicious fig wasps in viscous populations Understanding when interacting individuals should conflict or cooperate is an enduring topic in evolutionary ecology. Cooperation can be favoured through kin selection when those interacting are genetic relatives. Because high levels of relatedness can be generated in ‘viscous’ populations, characterized by limited dispersal, it has often been assumed that population viscosity will lead to cooperation. However, the same viscosity might also increase resource competition between relatives, and recent theory predicts that this might oppose the evolution of cooperation. The predicted and opposing influences of relatedness and competition can be difficult to disentangle in practice but a new comparative study on male–male conflicts in fig wasps shows that fighting is correlated with resource competition rather than with genetic relatedness1. Fig wasps develop within sycones, which are the closed inflorescences often called ‘figs’. The females are always winged but males of many species are wingless and nondispersing. Wingless males mate with females inside the sycone, before the females disperse. Male–male aggression varies greatly among species: in some, males have a ‘combat morphology’, consisting of armour and large
mandibles, whereas, in others, males have no structures for fighting or for defence. The average relatedness of competing males also varies greatly among species, mainly as a result of varying numbers of ‘foundress’ mothers laying eggs into each sycone (when more mothers contribute offspring, average offspring relatedness decreases). West et al. used sex ratio (a known correlate of foundress number) as an index of relatedness and quantified conflict levels across 25 species by assessing physical damage sustained by males during the mating period (scoring 0.5 points for loss of an antenna and the maximum of eight points for decapitation). Controlling for phylogeny, they found no relationship between relatedness and conflict level, but there was a negative correlation between injury level and the mean number of females (mating resource) developing in a fig.
This study provides the first good support for the prediction that lack of dispersal before resource competition counteracts possible benefits of cooperating with kin. Instead, conflict levels appear to be determined by the importance of fighting for (and mating with) any particular female: when females are rare (small brood sizes) fighting is at its most intense. West et al.’s paper is valuable not only because it tests the alternative predictions but also because it emphasizes the need for empiricists to assess competition, as well as relatedness. Moreover, the scale at which competition operates is important. For fig wasps, the key biological scale is at the sycone level: without male dispersal from the sycone, it appears that kin selection is cancelled by resource competition. 1 West, S.A. et al. (2001) Testing Hamilton’s rule with competition between relatives. Nature 409, 510–513
Ian C.W. Hardy [email protected] James M. Cook [email protected]
Faecal pellets and energy flow in rivers With the ever-increasing need for water abstraction and power generation, the attention of many researchers interested in freshwater systems has turned to river flow management. However, as a new paper by Björn Malmqvist, Roger Wotton and Yixin Zhang shows, interest in management of anthropogenic changes to ecosystems does not preclude advancement in basic science. Blackfly larvae (Diptera: Simuliidae) are ubiquitous in most freshwater ecosystems, and their feeding habits result in an unusual transformation of organic matter. Along with several other filter-feeding macroinvertebrates, blackfly larvae remove fine particulate organic matter, and even dissolved organic matter, from the water column and, as a consequence of somewhat poor digestion efficiency, produce large numbers of faecal pellets (FPs). In some
ways, this process is analogous to the amalgamation of organic matter into ‘marine snow’ by marine suspension feeders. However, in flowing water, the process of amalgamation results in increased retention of carbon, whereas in the marine environment and lakes, it results in increased loss of carbon from the upper layers. In streams and rivers, the suspended organic matter captured but not assimilated by the blackfly larvae would otherwise be carried downstream. The ‘recycled’ organic matter is heavier than its constituent parts and so sinks more rapidly, which, in turn, means that the organic matter contained in the FPs is available to the large contingent of detritus feeders found in flowing waters. Malmqvist et al. show that if these FPs were not produced, the potential energy loss of organic material from large river systems
could be immense. For instance, they estimated that for a single river in the period of April 1999 to August 1999, the average daily transport of organic matter in the form of FPs was 69.2 tonnes (dry mass), equivalent to 2.7 tonnes of carbon per day. The authors also sampled a river that had been dammed, and suggest that regulation of rivers by damming could considerably reduce the concentration of FPs in the water, either through negative impacts on blackfly populations via reduction of available habitat, or perhaps through reductions in flow, resulting in more rapid sedimentation of the FPs. However, in terms of carbon retention, these two processes have conflicting effects: loss of habitat means carbon is lost from the system, whereas reduced flow could result in increased carbon retention through increased sedimentation. Hence, the effect of damming on carbon flow is
http://tree.trends.com 0169–5347/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.
News & Comment
difficult to predict and likely to vary between and within rivers. If increased sedimentation of FPs turns out to be the main factor at work in regulated rivers, then the implications for the energy flow and functioning of these systems is unclear. Locally, increased settling of FPs into reservoirs might act as a sink – locking up the organic matter, and possibly contributing to increased methane release. Alternatively,
TRENDS in Ecology & Evolution Vol.16 No.5 May 2001
the flux of particulate organic matter available to detritivores in rivers might increase at the expense of those in estuarine systems. Either way, regulation of rivers would seem to be redirecting a large portion of the energy flow through river systems and should receive more attention, both from those interested in management of perturbed systems and those involved with basic ecosystem functions.
225
1 Malmqvist, B. et al. (2001) Suspension feeders transform massive amounts of seston in large northern rivers. Oikos 92, 35–43
Stuart Humphries [email protected] Graeme D. Ruxton [email protected]
Inbreeding in nature: brothers and sisters, do not unite! Inbreeding, or mating among related individuals, causes problems with viability and fertility in small captive populations. Young inbred ungulates often die in zoos, and the eggs of inbred fruit flies frequently fail to hatch in the laboratory. But what is the role of inbreeding in nature? Does it really affect patterns of population change in the wild, and could it even cause extinctions? These questions were the subject of hot debate in the 1970s. By the late 1980s, many ecologists seemed to agree that other ecological factors were more important than inbreeding in reducing population size and in extinguishing populations. However, we could now be witnessing the revival of interest in inbreeding as a factor to contend with. Whereas theoretical studies have demonstrated how the accumulation of mildly deleterious alleles can lower the fitness of small populations, experiments, both in the laboratory and in the wild, have demonstrated significant effects of inbreeding on the density of several animal and plant species. Observations in 1998 suggested that inbreeding increases the risk of population extinction in the butterfly Melitaea cinxia1. Some of the same authors now report experimental evidence2 that not only confirms the earlier results, but also demonstrates when and how inbreeding
exerts its harmful effect. Working on small populations of Melitaea cinxia in the laboratory, Nieminen et al. found that inbreeding lowered the proportion of eggs hatched and decreased the survival of overwintering larvae. The larvae of this butterfly spin communal winter nests, but inbreeding made them spin nests full of holes, which failed to protect them from the elements. And when Nieminen et al. introduced in- and outbred populations into the field, they found that inbred populations had a much higher risk of extinction than did outbred ones. This study suggests that inbreeding in Melitaea cinxia may reduce local survival to the point of local extinction. And as the species occurs as a set of classical
metapopulations, where the extinction of small local populations is counterbalanced by the colonization of empty sites, an increased risk of local extinction might ultimately threaten the survival of the species at a landscape level. The work of Nieminen et al. is clearly a case study on a single species with many special features. In their study region, Melitaea cinxia typically occurs as very small local populations, where inbreeding between close relatives is more the rule than an exception. This should keep us from generalizing the current results too broadly. But by identifying inbreeding as a key agent in the dynamics of at least this species, Nieminen et al. do offer an eye opener. With more and more species being split into smaller and more fragmented populations, perhaps inbreeding might soon be a threat to the survival of many wild populations? 1 Saccheri, I. et al. (1998) Inbreeding and extinction in a butterfly metapopulation. Nature 392, 491–494 2 Nieminen, M. et al. (2001) Experimental confirmation that inbreeding depression increases extinction risk in butterfly populations. Am. Nat. 157, 237–244
Tomas Roslin [email protected]
Landscape dynamics can accelerate metapopulation extinction In metapopulations, local populations can be founded in, and go extinct from, distinct patches of suitable habitat. Previously, metapopulation models assumed static landscapes, where suitable habitat patches neither were created nor went extinct. Dynamics and persistence of such metapopulations were governed by the interaction between the life history of species, which determines the rates of
local extinction and colonization, and landscape properties. Metapopulation persistence was more probable for a larger amount of suitable habitat, and especially so, if suitable patches were connected. However, for many species, landscapes are highly dynamic rather than static. Therefore, Juan Keymer and co-workers conducted a new study of metapopulation persistence with a dynamic landscape
model where suitability of habitat patches changes over time1. Keymer et al. confirm that metapopulation persistence depends on the life-history of a species and on the amount of available habitat. Instead of purely spatial patch connectivity, however, spatio-temporal patch connectivity now becomes important. So far, at low connectivity, local dispersal was not
http://tree.trends.com 0169–5347/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.